1. Field of the Invention
The present invention relates generally to terminal structures of superconducting cables including a cable core having a superconducting layer, and superconducting cable lines including the terminal structure, and particularly to terminal structures of superconducting cables helping to attach a ground wire to the superconducting layer while preventing an electrical insulation layer from having impaired insulating property, and lines including the terminal structure.
2. Description of the Background Art
As one of superconducting cables that have been produced using a superconductor layer formed of a Bi-based high-temperature superconducting tape for example, not only a single phase cable including a single cable core but a multiphase superconducting cable of multicore type that is produced by assembling a plurality of cable cores into one unit is also under development.
Referring to FIG. 5, this superconducting cable 100 includes three cable cores 102 twisted and housed in a thermal insulation pipe 101. Thermal insulation pipe 101 has an outer pipe 101a and an inner pipe 101b. This double pipe constructed of these outer pipe 101a and inner pipe 101b has a thermal insulation material (not shown) provided therein and a vacuum is produced within the double pipe.
These cable cores 102 each include, in the order starting from the innermost component, a former 200, a superconductor layer 201, an electrical insulation layer 202, a superconducting shield layer 203, and a protection layer 204. Superconductor layer 201 is constructed by winding superconducting wires around former 200 in a spiral manner in layers. Electrical insulation layer 202 is formed of wound insulation paper formed of polypropylene and kraft paper in lamination. Superconducting shield layer 203 is constructed by winding superconducting wires similar to that of superconductor layer 201 around electrical insulation layer 202 in a spiral manner. In this superconducting shield layer 203, in a steady state, a current is induced of substantially the same magnitude as and opposite in direction to a current flowing through superconductor layer 201. The induced current creates a magnetic field, which can cancel out a magnetic field generated from superconductor layer 201 and thus there is substantially no magnetic field leaking outside cable core 102. A space 103 formed between inner pipe 101b and each cable core 102 normally provides a path where a refrigerant flows. Thermal insulation pipe 101 has a radially outer portion provided with an anticorrosion layer 104 of polyvinyl chloride.
For normal conducting multiphase cables, by contrast, it is known that normally each cable core has a shield layer grounded to obtain a ground potential for each phase. This technique is described for example in Power Cable Technology Handbook, New Edition by Kihachiro Iizuka, Kabushiki Kaisha Denkishoin, Mar. 25, 1989, first edition, first copy, p. 645. The shield layer is grounded by directly attaching a ground wire to the shield layer and grounding the wire, connecting the wire to a grounded construction. The ground wire is often attached to the shield layer by bonding using typical solder having a melting point of approximately 190° C., compression-bonding, or the like.
A superconducting cable also requires having a superconducting shield layer processed, and grounding the superconducting shield layer is desired. If the superconducting cable has the superconducting shield layer grounded in the manner employed to ground a normal conducting cable's shield layer, however, a problem arises: First, a superconducting layer is formed of a superconducting wire which does not have a mechanical strength that can endure compression bonding. As such, such compression molding as employed for normal conducting cables cannot be employed. If the superconducting cable has the superconducting shield layer with a ground wire attached thereto by using the above-described, typical solder, which has a melting point higher than the temperature limit of the electrical insulation layer underlying the superconducting shield layer, the heat applied to melt the solder impairs the electrical insulation layer's insulating property.
Furthermore, directly attaching a ground wire to the superconducting shield layer of the superconducting cable may damage a superconducting wire configuring the superconducting shield layer and impair the electrically insulation layer's insulating property, as described above. Accordingly, rather than directly attaching a ground wire to a superconducting shield layer, indirectly attaching the ground wire to the superconducting shield layer is desired.